1. Field of the Invention
The present invention relates to synthetic-grass structures and in particular to the corresponding filling materials (referred to commonly by the term “infill”).
2. Description of the Related Art
Synthetic-grass structures usually comprise a substrate with a plurality of filiform formations extending from the substrate so as to simulate the sward of natural grass cover or turf. The particulate infill dispersed between said formations has the primary purpose of keeping the filiform formations themselves in a substantially upright condition.
Solutions of this type are described, for example, in U.S. Pat. No. 5,958,527 or EP-A-1 158 099. In particular, in the solution described in the latter document, which is filed in the name of the present applicant, the particulate filling material (infill) consists of a substantially homogeneous mass of granular plastic material.
Further information on said structures may be obtained from EP-A-1 319 753, which describes a method for removing said synthetic-grass structures with a view to the possible recovery of components, and again from the European patent applications 02029021.9 and 02425398.1, also these documents being, as likewise EP-A-1 319 753, filed in the name of the present applicant.
The synthetic-grass structures referred to previously are increasingly considered as a valid alternative to natural turf, in particular, for applications in which, for various reasons (environmental conditions, intense use, etc.) the upkeep of natural grass cover proves critical, also as regards the expenditure involved in said upkeep.
For example, it is known to use synthetic-grass structures of the type described for making sports fields, such as soccer pitches, American football pitches, tennis courts, and five-a-side football pitches. It is also possible to envisage the use of synthetic-grass structures of this type for laying athletics tracks.
In the production of the synthetic-grass cover of the type described, it is necessary to take into account various sorts of basic factors.
One aim of the invention is to create synthetic-grass cover that will have an appearance as close as possible to the appearance of natural turf. In other words, the aim is to prevent a synthetic-grass cover from revealing its nature too evidently. The aesthetic aspect, and in particular the chromatic appearance of synthetic-grass cover, may be influenced also by the nature and by the characteristics of the infill. In fact, even though in normal conditions of laying the material in question is to a large extent hidden from view by the filiform formations that simulate natural sward, the coloring of the infill contributes to the overall chromatic effect of synthetic-grass cover, a chromatic effect which, in the vast majority of cases, has the aim of simulating as faithfully as possible the chromatic effect of natural turf.
Other factors to be taken into account are linked to the wear of the synthetic-grass cover during use. In the absence, in fact, of the mechanism of re-growth of natural turf, synthetic-grass cover tends to wear out (or at least to be altered) in a non-uniform way according to the different conditions of use and of stress to which different areas of the flooring may be subjected. To provide an example in order to facilitate understanding, it may immediately be appreciated that, in the case of a flooring for a pitch for playing soccer, a midfield area and the areas of the goal tend to be particularly subject to stresses, and hence to wearing out to a more marked extent than the other areas, such as for example the areas near the corners of the field or immediately at the sides of a goal. It should then be taken into consideration that some types of stress may lead to the infill getting knocked out of place: it will suffice to consider typically the case of an athlete who is wearing shoes provided with studs or spikes and who exploits precisely these studs or spikes to exert a strong thrust forwards or a sharp deceleration. Usually, the filiform formations of the grassy cover do not exert an appreciable action of anchorage in regard to the infill when the infill is subjected to such an intense stress.
In this context, it is important that the artificial cover—and above all the infill—must enable rain water to run off easily, preventing any phenomena of waterlogging, formation of puddles, and any risk of the infill floating on the water.
Other factors to be taken into consideration are strictly linked to the laying of the synthetic-grass flooring. Common practice is to lay first the sheet substrate provided with the filiform formations that simulate the grassy sward of natural turf and subsequently to sow on said cover the particulate infill.
This operation carried out “in field” is of course open to various critical factors. Just to limit our considerations to a few examples, before proceeding to sow particulate infill it is usually preferable to carry out a prior intervention on the filiform formations so that the latter will, at the moment of sowing of the infill, be oriented in a substantially vertical direction, the aim being to have the particulate material gradually deposited starting from the lowest level adjacent to the ground. Again, the exact dosage of the amount of particulate material deposited per unit surface requires the availability of appropriate equipment and, in general, of skilled staff. In the case where the infill is a plastic material, there may then enter into play other factors, for example ones linked to the temperature at which the operation of sowing the infill is carried out.
Finally, a further set of factors to be taken into account is linked both to considerations of an economic nature and to considerations of environmental impact.
The first forms of synthetic-grass cover envisage in fact, in the majority of cases, the use of infill consisting of sand, i.e., a material which has undoubtedly the advantage of being inexpensive and readily available, but which presents numerous drawbacks in relation to all the factors of use and application considered previously. Furthermore, the use of sand (or of particulate infill with quite a high component of sand) negatively affects the biomechanical characteristics of the artificial grass cover and moreover exposes the person who happens to fall on the synthetic-grass flooring to serious risks of injury.
The use, as particulate infill for synthetic-grass flooring, of granular plastic materials practically overcomes all the drawbacks delineated above, but comes up against evident considerations of cost.
The above considerations may play a determining role if it is considered that the interest in the installation of synthetic-grass flooring may arise not only in statutory sports bodies or organizations of a professional or semi-professional character, but also in the sphere of amateur institutions.
For this reason there has been proposed and adopted the choice of using as particulate infill granular material obtained from recycled-tire material and in particular from automobile tires.
The term “recycled-tire material” comprises in itself a certain range of compositions. This consists, in the majority of cases, of elastomer compounds (for example SBR) with the addition, to a greater or lesser extent, of carbon-black fillers, these compounds possibly containing within them, according to commonly used recycling techniques, more or less extensive amounts of impurities. These may, for example, be metallic impurities deriving from metallic components and from the reinforcement fabric of tires when these have undergone cutting and shredding for producing the recycled granules.
The use of particulate recycled-tire material as infill for synthetic-grass cover thus presents undoubted advantages of an economic nature, it being a material that is readily available in large quantities and at a contained cost, since it is, above all, a material the disposal of which is to be facilitated. However, it presents rather important disadvantages from all the other points of view considered previously.
In the first place, the amply preponderant black coloring (due to the high content of carbon black of said recycled material) ends up, in the majority of applications, by adversely conditioning the overall chromatic appearance of the artificial grass cover. In addition, said black coloring involves a high absorption of solar radiation, with a consequent possible overheating of the synthetic-grass cover, until it reaches—in the said cover—a temperature in the region of 60–65° C., given a temperature of the air of 28–30° C.
In the second place, at least in certain materials recycled from tires, the phenomena of wear deriving from the mechanical stresses applied to the synthetic-grass cover during use may lead, in the areas subjected to high stress, to a further fragmentation of the granules and/or to ablation from the granule itself of microscopic particles having the character of a dust, which may get kicked up or rise in an undesirable way from the synthetic-grass structure. This moreover phenomenon occurs in a markedly differentiated way in different areas of the synthetic-grass flooring, according to the more or less intense conditions of mechanical stress.
The carbon black that is present and may be released from said infill is—as is well known—an excellent pigment. This means that anyone who accidentally happens to fall on the synthetic-grass cover which contains said infill, when he picks himself up will find that he has more or less extensive black patches on parts of his body and clothes involved in impact with the ground.
Again, a factor that is extremely negative, the dust of carbon black (and of metallic contaminants present possibly as residue) that is released by said infill is readily washed away by rain water into the underlying ground, with a pollutant effect, if it is also taken into account that the carcasses of tires may contain metallic components which might give rise to toxic residue or at least residue that is hardly compatible with the environment.
Finally, it is important that the material deriving from recycled tires, which is used as infill for a synthetic-grass structure, was initially formulated for an altogether different use and is characterized by a relatively low density (typically comprised between 0.9 and 1.1 g/cm3), combined with a high degree of resilience.
Both of these characteristics are unfavorable for use as infill for synthetic-grass floorings. The first of these characteristics (low density) favors in fact the possible floating of the infill on water, with consequent risk of undesirable displacement. The second of these characteristics (high resilience) means that the synthetic-grass cover proves too elastic not only for the tread and for the purposes of running but also as regards bouncing of a ball used for a particular sport.